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This invention relates to electro ceramic components and structures that accurately control surface potentials around electro ceramic components. Components constructed according to the invention can be Micro Electro Mechanical System (MEMS) devices, MEMS arrays, or other micromachined elements.
Conventional MEMS array structures comprise Silicon on Insulator (SOI) array structures associated with an electrode array disposed to interact with the MEMS actuatable elements. Electrostatic MEMS structures develop forces and torques between the actuatable elements and their corresponding electrodes. Conventional MEMS structures separate conductive surfaces with dielectrics. These dielectrics contribute to the mechanical operation of the device because accumulated charge distributions on their surfaces contribute to the electrostatic force and/or torque on the MEMS actuatable elements. One of the problems encountered is control of the surface potentials between the electrodes and control of the surface potentials on the insulators. Surface potentials on dielectric surfaces are prone to drift over time due to charge migration along dielectric surfaces between said electrodes. This can cause serious problems regarding repeatability of positioning. An issue that arises is that the potential of these surfaces is not controlled due to non-linear conduction across the surfaces of ions and charges that accumulate in an uncontrolled fashion. The conduction characteristics of these surfaces are inherently unstable due to sensitivity to temperature, moisture and other environmental factors. They can also be affected by electromagnetic radiation (light), which can be time dependent depending on the application, contributing to system crosstalk. The conductivity of these surfaces is also strongly affected by impurities and process steps and materials used in the deposition and etching of the surfaces. All of these factors combined contribute to a loss of control of the surface potentials that contribute to the forces and torques applied to the actuatable elements resulting in an unreliable and uncontrollable device.
What is needed is a solution that mitigates the effects of uncontrolled dielectric surface potentials so that the electrostatic forces and torques are determined solely by the voltages applied to the electrodes.
According to the invention, in an electrostatically controlled deflection apparatus, such as a MEMS array having cavities formed around electrodes and which is mounted directly on a dielectric or controllably resistive substrate in which are embedded electrostatic actuation electrodes disposed in alignment with the individual MEMS elements, a mechanism is provided to mitigate the effects of uncontrolled dielectric surface potentials between the MEMS elements and the electrostatic actuation electrodes, the mechanism being raised electrodes relative to the dielectric or controllably resistive surface of the substrate. The aspect ratio of the gaps between elements (element height to element separation ratio) is at least 0.1 and preferably at least 0.5 and preferably between 0.75 and 2.0 with a typical choice of about 1.0, assuming a surface fill factor of 50% or greater. Higher aspect ratios at these fill factors provide incremental marginal improvement.
In a specific embodiment, the substrate has electrode elements having a height of at least 15 microns for a separation between elements and between elements and side walls of no more than 150 microns, where the surface fill factor is at least 50%.
In a further specific and preferred embodiment, the substrate has electrode elements having a height of at least 150 microns for a separation between elements and between elements and side walls of no more than 150 microns, where the fill factor is approximately 75%. It has already been discovered that the saturated drift impacting tilt angle of actuatable MEMS elements is controllable to better than 150 microradians, or better than about one part in 500 over all possible input voltages up to a breakdown voltage of nearly 600 volts.
In a further specific embodiment, the potential on the dielectric surfaces on the substrate is controlled using a highly-resistive coating so that the surface potential between adjacent electrodes and between the electrodes and the actuatable element is determined by small but stable leakage currents between electrodes of different potentials. The leakage current is limited by material characteristics so that power dissipation levels and crosstalk between electrodes are mitigated, yet it permits enough current to flow to create stable, repeatable and temperature-independent and humidity-independent potential gradients along the surfaces to allow for highly accurate deflection of the MEMS actuatable elements.
In another embodiment, the entire dielectric substrate is allowed to be slightly conductive, that is, conductive with high resistivity. The surface potentials between electrodes are controlled without necessitating an additional deposition.
The invention will be better understood by reference to the following detailed description in connection with the accompanying illustrations.